Fascinated Journeys into Blue Light Isamu AKASAKI Meijo University and Nagoya University CONTENTS 1. Introduction 2. Creation of GaN single crystal with excellent quality 3. Development of GaN p-n junction Blue LEDs and Laser diodes 4. Summary 1/20
1. Introduction 2/20 Blue Light-Emitting Devices (LED, Laser diode) ［ A ］ Energy bandgap Eg ： >2.6 eV (< 480 nm) (Wide bandgap semiconductors) ［ B ］ Energy band structure ： Direct-transition type for conservation of electron momentum Conduction band electron Energy bandgap Eg Electron energy Excitation Light light (radiative recombination) (Positive) hole Valence band (Internal) photo-electric effect spontaneous emission Conservation of energy
1. Introduction 3/20 High-performance Blue LED and Laser diode [1] High-quality single crystal [2] p-n junction Depletion layer n type p type - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + electron hole Electron energy － ＋ Light emission Eg hole （ ~3V ） Applied forward voltage Electric current p-n junction LED p-n junction
1. Introduction 4/20 Candidate materials for Blue Light-Emitters in 1960s-’70s ZnSe GaN [A] Energy gap (Eg) 2.7 eV 3.4 eV [B] Energy band structure direct direct [1] Crystal growth straightforward too difficult Substrate GaAs sapphire Lattice mismatch 0.26 % 16 % [2] p-n junction not realized at that time few Number of researchers many Physical & chemical stability low high
1. Introduction 5/20 Candidate materials for Blue Light-Emitters in 1960s-’70s ZnSe GaN [A] Energy gap (Eg) 2.7 eV 3.4 eV [B] Energy band structure direct direct [1] Crystal growth straightforward too difficult Substrate GaAs sapphire Lattice mismatch 0.26 % 16 % [2] p-n junction not realized at that time few Number of researchers many Physical & chemical stability low high Chose GaN in 1973 at Matsushita Research Institute Tokyo ( MRIT ) because of toughness , wider direct Eg , and non-toxicity
1. Introduction 6/20 Started growth of GaN by MBE in 1973, and by HVPE in 1975 GaN MIS Blue LED by HVPE First as-grown highly n-type cathode SiO 2 film sapphire 1978 Epoxy dome h ν n-GaN n-GaN i-GaN (30 μ m) Ohmic MIS Blue LED i-GaN electrode (~1 μ m) The brightest Blue LED at that time, however, still weak and high operating voltage MIS Blue LED, not p-n junction LED at MRIT
1. Introduction 7/20 Potential of GaN at MRIT High-quality tiny crystallites 100 μ m Surface of GaN grown on sapphire by HVPE (1975-78) Tiny but high-quality crystallites embedded in HVPE-grown crystals Recognized the great potential of GaN Made up my mind to go back to the beginning; i.e. Crystal Growth in 1978
2. Creation of GaN single crystal with excellent quality 8/20 Crystal growth methods for GaN Hydride Vapor Phase Epitaxy (HVPE) H. P. Maruska and J. J. Tietjen: (1969). GaCl (g) + NH 3 (g) = GaN (s) + HCl (g) + H 2 (g) Issues: Susceptible to reverse reactions, Too fast growth rate Molecular Beam Epitaxy (MBE) I. Akasaki: (1974) (unpublished). 3 Ga (g) + NH 3 (g) = GaN (s) + H 2 (g) 2 Issues: Prone to nitrogen deficiency, Slow growth rate (at that time) Metalorganic Vapor Phase Epitaxy (MOVPE), (MOCVD) H. M. Manasevit et al: (1971). Ga(CH 3 ) 3 (g) + NH 3 (g) GaN (s) + 3CH 4 (g) Advantages: • No reverse reactions • Easy to control growth rate, alloy (AlGaN, GaInN) composition, and impurity-doping Decided to adopt MOVPE (1979) at MRIT
2. Creation of GaN single crystal with excellent quality 9/20 Started anew to MOVPE since 1981 at Nagoya University Improvements in MOVPE reactor and growth condition (1) (2) NH 3 +H 2 TMG+TMA+H 2 NH 3 +H 2 (1985) TMG+(TMA)+H 2 0.7~20cm/sec delivery tube Y. Koide Reactor substrate sapphire 110 cm/sec (1) guide susceptor RF coil Inclined substrate (2) Reactor design changed Exhaust Exhaust First MOVPE system (Handmade) 10 μ m Mixing TMG (TMA) with NH 3 just before the reactor inlet, and (1) High speed gas flow (2) Substrate inclined at a 45-degree angle GaN Suppressed the convective gas stream, and the adduct formation sapphire Bird-view Uniform growth, but not specular surface, still poor material quality SEM image by H. Amano
2. Creation of GaN single crystal with excellent quality 10/20 Growth on Growth on the same substrate a highly-mismatched substrate Interfacial energy σ defects GaN A A GaN mismatch GaN ~ 16% A B sapphire but not available Lattice Homoepitaxy Heteroepitaxy Lattice matching Huge lattice-mismatch For epitaxial growth, it is considered to be gospel to have a lattice matching: (e. g. Si on Si, GaAs on GaAs)
2. Creation of GaN single crystal with excellent quality 11/20 (3) Innovation in MOVPE growth method (1985) Low-temperature (LT-) buffer layer Sapphire(0001) NH 3 +H 2 LT-buffer (20~50 nm) ~500 o C 425 cm/sec TMG(TMA)+H 2 H. Amano (1) delivery tube GaN LT-buffer layer High-quality substrate (3) GaN ~ 1000 o C susceptor (2) RF coil LT-buffer newly- Direct growth developed (Common method) Exhaust Key technologies: (1) Much higher-speed gas flow (425 cm/sec) (2) Substrate inclined at a 45-degree angle (3) Deposition of thin AlN buffer layer at about 500 o C, before the growth of GaN single crystal at about 1000 o C
2. Creation of GaN single crystal with excellent quality 12/20 Creation of high-quality GaN (1985) Until 1985 Since the late 1985 GaN grown by MOVPE using LT-buffer GaN grown by HVPE GaN grown by MOVPE 100 μ m Sapphire (b) 2mm GaN island crystal Many cracks, pits Crack-free, pit-free Rough surface Specular surface Dislocations: > 10 11 cm -2 Dislocations: 10 8 -10 9 cm -2 Free electron conc. >10 19 cm -3 Free electron conc. < 10 16 cm -3 Electron mobility: ~20 cm 2 /V ･ s Electron mobility: ~700 cm 2 /V ･ s Weak luminescence Intense luminescence Crystal quality, electrical property, and luminescence property were dramatically improved at the same time
2. Creation of GaN single crystal with excellent quality 13/20 Growth model of GaN using LT-buffer layer Surface (SEM) images Growth model Mixture both amorphous & (1) As-deposited fine crystallites of AlN K. Hiramatsu LT-AlN buffer layer LT-buffer layer Sapphire (2) 5 min Increase GaN thickness GaN growth Lateral growth of GaN Direct growth for 60 min. dominates GaN (No LT-buffer) (3) 10 min GaN growth LT-buffer layer Surface (SEM) images Growth model Sapphire (4) 20 min GaN growth GaN layer of excellent quality GaN island (5) 60 min GaN growth LT-buffer layer 1 μ m Sapphire GaN island Sapphire
3. Development of GaN p-n junction Blue LEDs and Laser diodes 14/20 Realization of p-type GaN, AlGaN, and GaInN 1986 High-quality GaN using LT-buffer layer Basic Technology (Low residual impurities) 1988 Found greatly enhanced blue emission of Zn doped GaN by electron irradiation (LEEBI) H. Amano 1989 Doped Mg using CP 2 Mg and electron irradiation High-quality Mg-doped Achieved the first p-type GaN GaN subjected to LEEBI 1991 p-type AlGaN (C 5 H 5 ) 2 Mg 1995 p-type GaInN M. Kito
3. Development of GaN p-n junction Blue LEDs and Laser diodes 15/20 The world’s first GaN p-n junction blue LED (1989) M. Kito H. Amano p-n LED MIS LED Current Voltage GaN p-n junction Blue LED I-V curves
3. Development of GaN p-n junction Blue LEDs and Laser diodes 16/20 Conductivity control of n-type GaN, AlGaN 1986 High-quality GaN using LT-buffer layer Basic Technology (Low residual impurities) Electron concentration [cm -3 ] 10 19 1989 Doped Si into high-quality GaN using SiH 4 Achieved conductivity control of n-type GaN 10 18 1991 n-type AlGaN 10 17 Allowed the use of heterostructure and quantum well in the design of more efficient p-n junction light- emitting structures 10 16 1 10 100 SiH 4 flow rate [sccm]
3. Development of GaN p-n junction Blue LEDs and Laser diodes 17/20 GaN-based laser Stimulated emission by Stimulated emission by UV (376 nm) Laser diode optical pumping (1990) current injection (1995) (1996) Room 388 nm AlGaN/GaN/GaInN temp. EL intensity (arb. units) Integrated light intensity (arb. units) ・ Emission intensity By current ・ injection EL intensity Current (mA) 3.0kA/cm 2 0 AlGaN/GaN/GaInN quantum well Room temp. 1.5kA/cm 2 ~ x 50 0 300 400 500 600 700 360 370 380 390 3.3 3.4 3.5 Wavelength (nm) Wavelength (nm) Photon energy (eV) On the basis of the technologies of LT-buffer layer and p-n junction heterostructures, GaN-based lasers were achieved
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